Embodiments of the present disclosure are directed to an electron beam imaging/inspection apparatus having an electron source device to direct flood electrons on a sample immediately before image acquisition or inspection. The apparatus comprises a first device configured to charge a sample in a first mode, wherein the first device includes an electron source configured to provide a flood beam of charged particles to a first area of the sample. The apparatus also comprises a second device configured to generate a primary beam of electrons and characterize an interaction between the primary beam and a second area of the sample within the first area in a second mode. The apparatus is configured to switch from the first mode to the second mode less than 1 second.
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1. An apparatus, comprising:
a first device configured to charge a sample in a first mode, wherein the first device includes an electron source configured to provide a flood beam of charged particles to a first area of the sample; and
a second device configured to generate a primary beam of electrons and characterize an interaction between the primary beam and a second area of the sample within the first area in a second mode, wherein the apparatus is configured to switch from the first mode to the second mode in less than 1 second.
24. A method, comprising:
imaging a first area of a sample within a first region of the sample with a primary beam of electrons from a first device;
charging the first area of a sample with a flood beam of charged particles from a second device, wherein the second device includes an electron source; and
imaging a second area of the sample within the first area with the primary beam of electrons from the first device, wherein a time between ending imaging of the first area and beginning imaging of the second area is less than one second.
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This application claims the priority benefit of commonly-assigned, co-pending U.S. provisional patent application No. 62/062,642, filed Oct. 10, 2014, entitled “Inspection Site Preparation”, the entire disclosures of which are incorporated herein by reference.
U.S. Pat. Nos. 8,748,845; 8,541,060; 8,329,138; 8,110,814; 7,928,384; 8,013,311; 7,335,879; 6,633,174; 6,569,654; and 6,433,561 are all related to the present disclosure. All of these patents are incorporated herein by reference in their entirety.
Aspects of the present disclosure generally relate to imaging and inspection systems, and more particularly to apparatus and methods for charge control on a sample and optimization of image quality in imaging and inspection systems.
Electron beam imaging requires an even charge distribution on a sample area in order to properly represent image contrast. A flood gun is used to neutralize initial charges which are carried over upon transfer of the sample to the imaging system by flooding the sample area with energetic electrons. Such flooding also equalizes charges that build up on the sample during imaging from the inspection beam. In some applications, e.g., semiconductor wafer imaging with a scanning electron microscope (SEM), the flood gun is used only during the acquisition of swath images. Frame imaging uses the electron gun to do both flooding and inspection.
The flood gun is very similar to the electron gun used in the electron beam imaging system. The main difference is that instead of emitting a focused beam concentrated on a small area (e.g., a few square microns or less), the flood gun sprays a wide beam of electrons, over a much larger area, e.g., about 1 square centimeter. The electrons from the flood gun approach perpendicular to the sample surface, flooding the sample area. Typical flood gun hardware includes a high current filament which heats a metal tip, causing emission of electrons toward an extractor electrode. The extractor controls the number of electrons which are emitted towards the wafer and can be used to turn on or turn off (blank) the beam. A Wehnelt electrode positioned close to the wafer surface creates an electric field to control the number of electrons which stay on the wafer. During inspection, the wafer floats at a potential controlled by a power supply.
Flooding and swathing typically commonly done sequentially but may be done simultaneously. Flooding and swathing generally operate on different areas of the sample. In swathing, the sample is imaged by rastering the primary beam in one direction (e.g., the X-direction) while the sample translates in a perpendicular direction (e.g., the Y-direction). By way of example, during a wafer inspection, a stage moves the sample such that an area of the sample is flooded before it is inspected. Flooding is typically either on or off for the entire duration of a swath. Because flooding covers a significantly larger area than a swath, flooding may be turned off, e.g., at least every other swath. To prolong the life of the flood gun, the flood gun beam may be blanked, e.g., by applying a suitable voltage to the extractor, when not needed.
It is within this context that aspects of the present disclosure arise.
Objects and advantages of aspects of the present disclosure will become apparent upon reading the following detailed description and upon reference to the accompanying drawings in which:
According to aspects of the present disclosure, an electron beam imaging/inspection apparatus has an electron source device to direct flood electrons on a sample immediately before image acquisition or inspection. The apparatus comprises a first device configured to flood a portion of a sample with charge in a first mode. The first device includes an electron source configured to provide a flood beam of charged particles to a first area of the sample. The apparatus also comprises a second device configured to generate a primary beam of electrons and characterize an interaction between the primary beam and a second area of the sample within the first area in a second mode. The apparatus is configured to switch from the first mode to the second mode in less than 1 second.
In some implementations, the first device is a flood gun.
In some implementations, the second device is a scanning electron microscope (SEM), a critical dimension scanning electron microscope (CD-SEM), e-beam inspection, e-beam review or e-beam lithography system.
In some implementation, the electron source for the first device includes an initiating radiation source, a micro-channel plate and an extractor, the initiating radiation source being configured to initiate generation of charged particles by the micro-channel plate, and the extractor being configured to extract the charged particles and form the flood beam.
In some implementation, the first device further comprises a blanking electrode configured to turn off the first device.
In some implementation, at least a portion of the flood beam is coaxial with the primary beam.
In some implementation, the first device is configured to provide the flood beam coaxially with the primary beam.
Another aspect of the present disclosure provides a method comprising charging a first area of a sample as a first mode with a flood beam of charged particles by a first device, wherein the first device includes an electron source; imaging a second area of the sample within the first area as a second mode with a primary beam of electrons by a second device; and switching from the first mode to the second mode in less than 1 second, more preferably, less than 0.1 second, still more preferably less than 0.01 second, and even more preferably less than 1 millisecond. It is desirable to switch rapidly between flood and image to minimize charge dissipation due to fast time constant discharge mechanisms.
Another aspect of the present disclosure provides an apparatus comprising an initiating radiation source; a micro-channel plate, wherein the initiating radiation source is configured to initiate generation of charged particles by the micro-channel plate; and an extractor configured to extract the charged particles and form a beam of the charged particles.
Although the following detailed description contains many specific details for the purposes of illustration, anyone of ordinary skill in the art will appreciate that many variations and alterations to the following details are within the scope of the present disclosure. Accordingly, the exemplary embodiments of the invention described below are set forth without any loss of generality to, and without imposing limitations upon, the claimed invention.
In a conventional scanning electron microscopy, a beam of electrons is scanned over a sample (e.g., a semiconductor wafer). Multiple raster scans are typically performed over a small area of the sample. The beam of electrons either interact with the sample and cause an emission of secondary electrons or bounce off the sample as backscattered electrons. The secondary electrons and/or backscattered electrons are then detected by a detector that is coupled with a computer system. The computer system generates an image that is stored and/or displayed on the computer system.
Typically a certain amount of charge is required to provide a satisfactory image. This quantity of charge helps bring out contrast in features of the sample. On a sample made of substantially insulative material (e.g., a semiconductor or semiconductor oxide or nitride), performing one or more scans over a small area may cause the sample to accumulate excess positive or negative charge in the image area relative to the rest of the sample. Excess positive charge may form a significant potential barrier to secondary electrons and result in fewer secondary electrons reaching the detector. Thus, the image area is likely to appear darker. On the other hand, excess negative charge may increase the collection of secondary electrons causing the image to saturate.
Excess charge remaining from a previous viewing or processing operation may therefore cause distortion. One solution used in SEM devices is to provide a flood gun in a vacuum chamber close to the imaging column and flood the sample with charged particles from the flood gun at a time separate from the inspection. Thus, flooding equalizes the charge appearing across the sample, thus enhancing the voltage contrast images.
Specifically, the flood gun may be a thermionic electron gun that uses one or more tungsten filaments as an electron source. In the thermionic electron gun, electrons are emitted from a heated filament and then accelerated towards an anode. A thermionic electron gun can deliver a total electron current of up to 100 μA. Another way to get emission is to decrease the “work function (energy barrier)” that limits emission. Materials with lower work functions emit at lower temperatures (e.g., Lanthanum Hexaboride (LaB6)). Unfortunately, LaB6 emitters cannot be used in certain applications because they tend to produce Boron trioxide, an insulator, which gets deposited in places in the chamber where it is not desired. Another common type of electron source in use today is a field emission sources. This type of source relies on a strong electric field at one or more emitters to produce emission at much lower temperature than a thermionic emission source. Typical field emission sources have one or more electrically conducting sharp points located proximate an extractor. A voltage between the sharp point(s) and the extractor produces a large electric field in the vicinity of the point due to the small radius of the point. As a result of the large electric field, electrons can quantum mechanically tunnel from the conduction band into the vacuum thereby producing electron emission.
In a common SEM operation, when a charging contact layer is formed as a result of exposure of the sample to the imaging beam, a stage moves the sample from under the SEM and takes it to a separate location under the flood gun. The flood gun irradiates the sample for a few minutes, e.g., up to 20 minutes. After flooding the wafer, the stage moves to the SEM and begins imaging.
One drawback to this flooding procedure is the long times involved in flooding, moving the sample to and from the SEM and long image acquisition times. Specifically, the stage including the entire sample has to be moved to the area of the flood gun. In order to accomplish the flooding, inspection with the SEM must stop to permit movement of the sample to the area of the flood gun. This dramatically increases the overall time required for the inspection since movement and flooding of the sample may take 20 minutes or more to complete. This produces an equally dramatic decrease in the throughput for the inspection process. Typically a full inspection of a sample will require hundreds of scan lines across the sample and the dissipation of charge may be required after only a few scan lines have been completed. The total time required for a sample to be inspected therefore is the sum of the separate intervals for charge dissipation (or pre-charging) and inspection. In addition, if imaging is performed for a long time, images acquired at the end of the run may be visibly worse than those from the beginning.
Consequently, what is needed is a method or apparatus that facilitates charge control with a high current density electron dose on a sample without requiring removal of the sample from the inspection beam or otherwise requiring inspection beam operation to be interrupted. Aspects of the present disclosure include an electron beam imaging apparatus (e.g., SEM) having an electron source device (e.g., flood gun) to direct flood electrons along the primary axis of an inspection/imaging beam in the imaging apparatus. Aspects of the present disclosure allow a sample to be charged in situ immediately before image acquisition.
Apparatus
Embodiments of the present disclosure may be implemented within any suitable measurement/inspection device that directs charged particles towards a sample and then detects emitted particles from the sample.
The apparatus 100 includes an imaging/inspection device and a flood gun integrated with the imaging/inspection device as shown in
By way of example and not by way of limitation, the sample 114 may be a wafer, reticle or thin film head substrate. In addition, a detector 116 is arranged to detect and collect secondary electrons (and/or backscattered electrons) emitted from the sample for further processing and analysis. Optionally, an energy filter (not shown) may be included to protect the detector 116 during flooding. In one embodiment, the energy filter may be applied using a negatively-biased electrostatic grid in front of a detector. In another embodiment, the energy filter is applied using an omega filter in front of a detector. In another embodiment, the energy filter may be applied using an electrostatic or magnetic deflector in front of a detector. In yet another embodiment, a voltage may be applied to the sample 114 to control the landing energy of the electrons in the beam 106. Additionally, a flood gun 200 is incorporated into the column 102 and is configured to provide a flood beam coaxially with the primary beam 106 as shown in
The primary electron gun 104, condensing lenses 108, scanning coils 110, objective lens 112, stage 115, detector 116, and flood gun 200 may be operably coupled to a controller 120, which may include power supplies configured to provide voltages and or currents to these elements in response to hardware or software instructions. The controller 120 may include a special purpose computer or a general purpose computer configured to operate as a special purpose computer upon execution of code instructions 122, which may be stored in a computer memory or other non-transitory computer readable medium.
According to aspects of the present disclosure, the apparatus 100 may be used to implement charge control during sample inspection according to an inventive method. According to the method a first area of the sample 114 within a first region of the sample may be imaged with the primary beam 106 of electrons from the primary column 102. The first area of the sample may then be charged with a beam of charged particles from the flood gun 200. A second area of the sample within the first area may then be imaged with the primary beam of electrons from the primary column 102. The configuration of the apparatus 100 allows a time between ending imaging of the first area and beginning imaging of the second area to be less than one second.
Method
Aspects of the present disclosure include implementations in which flooding and swathing occur simultaneously, but on different areas of the sample. In swathing, the sample is imaged by rastering the primary beam 106 in one direction (e.g., the X-direction) while the sample 114 translates in a perpendicular direction (e.g., the Y-direction) and a swath image is taken across a width of the sample. By way of example, during a wafer inspection, the stage 115 may move the sample 114 while the flood gun 200 provides a flood beam such that an area of the sample being swathed is flooded before it is inspected with the primary beam. To prolong the life of the flood gun, the flood gun beam may be blanked, e.g., by applying a suitable voltage to the extractor, when not needed.
Alternative Implementations
Aspects of the present disclosure are not limited to implementations involving the apparatus 100 of
An apparatus according to the above embodiments of the present disclosure may provide a large area, high density electron flood on the sample just moments before acquiring image data from any location within the flood area. Specifically, a sample 114 is positioned on a stage (not shown) to allow the primary beam 106 and the flood beam 212 to move over the sample. In other embodiments, the two beams remain stationary while the stage moves the sample underneath them. The apparatus may begin operation in a flood mode. In this mode, the electron source 210 of the flood gun 200 produces a flood beam of charged particles (i.e., the flood electrons). The flood beam charges the surface of the flood area to a certain potential while the stage settles. It is noted that the landing energy of the flood beam of charged particles may be set to any value. In some implementation, the landing energy may be set low for charge cancellation.
A voltage may be applied to the sample 114 to control the landing energy of the electrons from the flood gun 200. The landing energy of the flood electrons may vary over any suitable range. For example, in some implementations, the landing energy may be set to be the same as that of the primary beam 106. Alternatively, the landing energy of the flood beam may be substantially different from that of the primary beam. For example, in some implementations the landing energy may be set very high to charge certain features on the sample, e.g., buried features, for voltage contrast (VC) mapping. This allows for easier imaging of buried features by mapping the electric field contrast of the sample 114 after charging by the flood gun 200. In one embodiment, the flood gun 200 may deliver a total electron current of few μA to few mA into a flood area of 1 mm2 to 100 mm2 on the sample by pulsing the electron source for about 1 μs to about 1 ms. The duration of the flood time is determined by the desired surface charge density deposited and how fast the flood beam may be switched on and off. The amount of charge deposited may be determined by the surface potential developed during flooding. Therefore any suitable mechanism for obtaining a surface charge value of a portion of the sample may be utilized. By way of example and not by way of limitation, an electrostatic voltmeter or Kelvin probe may be used.
After flooding, the apparatus is switched to the imaging mode and image acquisition begins immediately anywhere in the flood area. According to aspects of the present disclosure, it can take less than 1 second from the end of an image acquisition to start and complete the flood mode and switch back to the start of a subsequent image mode. More preferably, this can be done in less than 0.1 second, still more preferably in less than 0.01 second, and even more preferably in less than 1 millisecond. This switch time depends partly on the flood time, which preferably takes about 1 μs to 1 ms, and partly on the settling time of the imaging system following the switch from flood to imaging mode. In some modes of operation, flooding may take place in conjunction with movement of the sample 114 by the stage 115 between image acquisition operations. Stage movement may take less than 200 milliseconds and the flood gun 200 may be activated, e.g., during the last few milliseconds of movement.
Image acquisition may take longer than flooding, e.g., from about 0.2 seconds to about 1 second. During operation in the imaging mode, the primary beam from the imaging/inspection device strikes an imaging area within the flood area and causes the emission of secondary electrons or the release of backscattered electrons from the image area. The secondary electrons (and/or the backscattered electrons) are then detected by the detector 116 for further process and analysis. The primary beam may move to another image area within the flood area for image acquisition. Once image acquisition in the flood area is done, the stage moves to next location of interest. The apparatus then switches to the flood mode and the process may start over again.
The electron source of flood gun 300 includes an initiating radiation source 312, a micro-channel plate (MCP) 314 and an extractor grid (not shown). The initiating radiation source is configured to initiate generation of charged particles by the MCP plate by supplying radiation that to the MCP 314 that initiates emission of electrons, e.g., by photoemission. The initiating radiation source 312 may be a light source of appropriate wavelength for the photo cathode of the MCP. By way of example and not by way of limitation, the initiation radiation source 312 includes an ultraviolet light emitting diode (LED) or a laser. The secondary electrons from the interaction between the radiation and the MCP are then extracted by the extractor grid. A condenser electrode (not shown) may be included to focus the charged particles extracted by the extractor grid and thereby forming a flood beam of charged particles. The MCP 314 can be turned on and off rapidly switching the initiating radiation source on and off. Alternatively, this may be accomplished by high speed voltage supplies 315a-c while the initiating radiation source 312 is turned on. It is noted that the flood current is adjustable with the MCP gain. In some embodiments, there are a stack of MCPs for additional gains.
Aspects of the present disclosure are not limited to implementations in which the flood beam source is coaxial with the primary beam source, as in
Aspects of the present disclosure include implementations in which the flood beam source is not coaxial with the primary beam but the flood beam and primary beam are at least partly coaxial. For example,
According to an aspect of the present disclosure, an electron beam imaging system having a flood gun may operate in an in-situ closed-loop flood mode to terminate flooding. Specifically, as illustrated with respect to
The flood gun 900 may be any type of flood gun described herein or a conventional flood gun. The flood gun 900 may be configured so that part of the flood beam 901 is coaxial with a primary beam 106 for a charged particle beam imaging system (e.g., a scanning electron microscope) over a portion of the primary beam path proximate a sample 114. A deflector (not shown), e.g., a Wien filter or electrostatic deflector plates, may steer the flood beam 901 into coaxial alignment with the primary beam 106 proximate the sample 114. The flood detector is configured to detect secondary electrons 903 and backscattered electrons from the sample 114 due to the flood beam 901. The imaging system may include a separate detector (not shown) to detect secondary electrons emitted from the sample during imaging.
The flood detector energy filter 910 may be a simple grid placed between the sample 114 and the flood detector 915. A voltage from a power supply 912 is applied to the grid repels electrons having a total energy less than some energy that depends on the grid voltage. The grid may have a small aperture that allows the primary beam 106 a clear path to pass through the energy filter 910 unimpeded during imaging operation.
The illustrated system may operate as follows. The primary beam 106 is switched off prior to flooding. The flood detector energy filter 910 is set to a voltage corresponding to a desired surface charge voltage added to the bias voltage applied to the sample wafer bias(WB (if any) For example if the desired surface charge voltage is +20V, a voltage of WB+20V would be applied to the energy filter 910. The controller 905 may trigger the flood gun 900 to begin an in-situ flood pulse. As the flood pulse begins the flood beam 901 charges the sample and the sample voltage begins to change as a result of the charging. The sample voltage could rise or fall but for the purposes of example, consider a case where the sample voltage rises. As the sample voltage rises secondary electrons and backscattered electrons leave the sample 114 with progressively lower total energy.
When the surface charge voltage reaches the desired value (e.g., +20V in this example) the secondary electron signal at the flood detector 915 begins to get cutoff by the energy filter 910. The energy filter 910 does not need to be particularly accurate so long as it is sufficiently precise. The trigger circuit 920 detects the drop in signal from the flood detector and triggers the flood gun controller 905 to end to the flood pulse.
If the flood beam current is sufficiently large, the secondary electron and backscattered electron yield from the sample during flooding may be larger than 100 microamps (μA). Even of the flood detector collects only a portion the secondary and backscattered electrons this should still be sufficient for the flood detector signal to have an adequate signal to noise ratio (SNR) with little gain.
Operation in an in-situ closed-loop mode allows for very fast flooding. If flooding can be done sufficiently fast, e.g., much less than 1 μs, it may be possible to start and finish flooding the sample as the primary beam (or beam steering voltage/current, if the beam is turned off) flies back across the imaged portion of the sample at the end of a line of an imaging scan with the primary beam in the x-direction before the primary beam begins the next line scan of the image.
According to an aspect of the present disclosure, an electron beam imaging system having a flood gun may operate in an alternative in-situ closed-loop to measure the voltage applied to the energy filter 910 while flooding. By reading back the value of the energy filter voltage as the sample is flooded, one can track the surface potential value and rate of change of surface voltage. This can be used to identify large area defects, for instance, a contact array with excessive leakage.
In operation, the energy filter 910 is biased to some voltage e.g., −5V, with respect to the surface potential of the sample. This causes the energy filter to “cut off”, or reject, all secondary electrons (SE) with less than 5 eV of initial energy. In this example only electrons with greater than 5 eV energy would make it to the detector 915. Consequently, the detector current to the gated integrator 930 is a value corresponding to 5 eV and above electrons. The gated integrator includes a summing junction that subtracts a DC current set to the expected value of the detector current. The output of the summing junction feeds the integrator. When the detector current equals the reference current no voltage is integrated. When the detector current is different from the reference current, voltage is added or subtracted. The output of the integrator is added to the energy filter voltage. If, e.g., the flood beam 901 charges the sample 114 positively, the detector current will begin to decrease causing a mismatch with the reference current and the integrator 930 will produce a digital output that adjusts the energy filter voltage power supply 912 to follow the surface potential of the sample 114. This signal may be read by a system controller (not shown) to track the surface potential.
By way of example, and not by way of limitation,
The rate of change of the energy filter voltage may be monitored and compared to a stored reference to determine if an area defect exists on the wafer. Alternatively, the flood beam may charge an area of the sample to saturation and the corresponding energy filter voltage may be compared to a reference to determine if an area defect exists on the wafer.
Aspects of the present disclosure facilitate in situ charge control on a sample in conjunction with charged particle beam inspection of the sample. By supplying a high current density electron dose to a sample charge may be controlled without requiring removal of the sample from the inspection beam or otherwise requiring inspection beam operation to be interrupted for long periods of time. These advantages allow for greater throughput of samples, e.g., semiconductor wafers, in a production environment.
While the above is a complete description of the preferred embodiment of the present invention, it is possible to use various alternatives, modifications and equivalents. Therefore, the scope of the present invention should be determined not with reference to the above description but should, instead, be determined with reference to the appended claims, along with their full scope of equivalents. Any feature, whether preferred or not, may be combined with any other feature, whether preferred or not. In the claims that follow, the indefinite article “A”, or “An” refers to a quantity of one or more of the item following the article, except where expressly stated otherwise. The appended claims are not to be interpreted as including means-plus-function limitations, unless such a limitation is explicitly recited in a given claim using the phrase “means for.”
McCord, Mark, Sears, Christopher, Young, Scott, Simmons, Richard, Masnaghetti, Douglas, Stanke, Fred E
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